In known heat exchangers used with gas heated devices, the opposite walls of the flue duct forming the extension of the combustion chamber are parallel to each other, i.e. the cross-sectional area of said duct is practically constant. There are solutions, where, at the very most, sudden changes occur in the cross-sectional area of the flue duct.
In some known constructions one or, in some arrangements, both media are passed between helical-shape ribs arranged in an annular duct. By this method, the conditions of heat transfer are somewhat improved by the ribs, but at the same time the flow-resistance is increased considerably.
In the arrangement described in the HU-PS No. 181.107 one medium flows in tubes, the other is passed between the ribs fixed to the tubes. The thin-sheet ribs attached to the spacers between the tubes improve the heat transfer, but increase the flow-resistance.
In the heat exchangers described in the DE-PS No. 2,343.007 two ducts are provided for the media taking part in the heat-exchange. The walls confining the ducts are essentially parallel, corrugated sheets.
The known solutions exhibit thermodynamically two fundamental disadvantages. One of these may be summarized as follows: The flue gases cool down while ascending in the duct provided for them, consequently their volumetric flow and also their flow velocity decreases. Since the heat transfer coefficient is proportional with some power of the flow velocity, so the heat transfer coefficient also diminishes together with the heat flux density valid for the wall of the duct.
The other disadvantage is the following: As already mentioned the flue gases, while ascending in the flue duct, cool down and with most gas heated devices the heat absorbing medium also while ascending along the device is heated up. Thus the difference between the temperatures of the two media rapidly decrease while ascending.
Due to the reduction of the temperature difference also the transferred heat is reduced. To compensate this effect the surface participating in the process of heat exchange has to be increased; this however leads to the increase of the size and weight of the device.
By the invention the outlined deficiencies and drawbacks of the known heat exchanger constructions can be eliminated.
The aim to be accomplished by the heat exchanger according to the invention has been to prevent any reduction of the heat transfer coefficient or at least to minimize it.